As a free safety, Fencik played twelve seasons with the Chicago Bears and at the time of his retirement was their all-time leader in interceptions and total tackles. He was the team's defensive captain through the 1980s including 1985 Super Bowl championship season. He made two Pro Bowl appearances (1980, 1981). He was also awarded a gold record and a platinum video award for the 1985 Super Bowl Shuffle.

Fencik finished his career with 38 interceptions, which he returned for 488 yards and a touchdown. He also recorded 2 sacks and recovered 14 fumbles, returning them for 65 yards.

Fencik along with Doug Plank were dubbed "The Hit Men", a fact referenced by Fencik in The Super Bowl Shuffle.

Following his football career, Fencik has worked in the finance industry.